Most key telephone sets are equipped with a bell or other audio indicator and some sets are also equipped with a visual indicator for indicating that the telephone set is ringing. Visual indicators are sometimes used to indicate that a message is waiting. This feature is often used by hotels having a private branch exchange (PBX). For example, if a hotel attendant wishes to alert a hotel guest that a message is waiting, the PBX may be programmed to switch on a message waiting indicator (MWI) on the recipient's telephone set once the handset is on-hook. A neon lamp, which is set to flash periodically is usually used.
With the advent of voice mail, public telephone subscribers may leave messages for other subscribers thereby invoking a similar scheme wherein a visual indicator indicates that one or more messages is waiting.
Typically these visual indicators are mounted in the housing of the telephone set and are comprised of a circuit including a neon lamp in series with a resistor coupled to the tip and ring terminals of a subscriber telephone line. The neon lamp provides the triggering threshold and the visual indication when the telephone set is ringing or when a message waiting signal is applied to the line. Although this approach is very simple, it has several shortcomings:
a) Neon lamps usually do not have precise firing points as the triggering threshold or firing point often varies from 80 to 130 volts. PA1 b) The extinguishing voltage can be as much as 45 volts below the firing point. In telephone lines where the nominal voltage is 48 volts DC, the neon lamp may inadvertently stay on after the message waiting voltage (MWV) or ringing signal is removed. PA1 c) Once the neon lamp has fired, there is no current control in the circuit and the circuit resistance is approximately the resistance of the series resistor. Varying the current causes varying brightness of the neon lamp; too much current can cause the lamp to burn out prematurely. Furthermore, the triggering of one neon lamp very often prevents the neon lamp in a second telephone set on the same telephone line from firing. PA1 d) The firing and extinguishing characteristics of a neon lamp changes with prolonged usage.